The Epidemiology Branch is conducting a number of birth defect studies in collaboration with the Health Research Board and Trinity College, Dublin, Ireland. The main objective of these studies is to determine the relationship between folate and birth defects. The birth defects studied to date are neural tube defects (NTDs), oral clefts, congenital heart defects, Down syndrome and omphalocele. These studies focus on biochemical factors in the area of folate metabolism, and on genetic mutations in folate related genes associated with birth defects. Recent work has expanded to include the biochemical pathways related to birth defects. Neural tube defects (NTDs) are common birth defects (1 in 1000 pregnancies in the US and Europe) that have complex origins, including environmental and genetic factors. A low level of maternal folate is one well-established risk factor, with maternal periconceptional folic acid supplementation reducing the occurrence of NTD pregnancies by 50-70%. This past year one focus of our work has been the public health problem of preventing NTDs. Dr. Mills and Dr. Molloy have published a commentary and an editorial on the strategies for prevention, specifically food fortification. The first publication has discussed the strategy of estimating optimal red cell folate to generate a target for fortification concentrations in women of childbearing age. This work showed the limitations of such an approach, notably that the populations used for modeling were not representative and the data did not take into account genetic factors and the limitations of models vs. actual data. This work confirmed the importance of the current fortification system and noted that case based studies have shown that the current fortification regimen in the US is very successful in preventing NTDs. The second publication dealt with the merits of folic acid fortification, showing that the risk of exacerbating vitamin B12 deficiency has not been proven and was unlikely to be a serious problem. This work supports the use of mandatory folic acid fortification. From a public health perspective, this is crucial because much of the world's population still does not receive folic acid to prevent NTDs. A second major focus of our group's work has been the folate and vitamin B12 biochemistry that underlies NTD risk. We demonstrated that FUT2 rs601338 genotype influences the glycosylation of haptocorrin. We then developed an experimental model demonstrating that holoHC is transported into cultured hepatic cells (HepG2) via the asialoglycoprotein receptor (ASGR). Our data challenge current published hypotheses on the influence of genetic variation on this clinically important measure and are consistent with a model in which FUT2 rs601338 influences holoHC by altering haptocorrin glycosylation, whereas B12 bound to non-glycosylated transcobalamin (i.e. holoTC) is not affected. Our findings explain some of the observed disparity between use of total B12 or holoTC as first-line clinical tests of vitamin B12 status. Formate is critical to folate metabolism as the donor of one carbon groups. Yet little is known about circulating formate concentrations or their determinants. Serum formate was measured in 1701 participants from the Trinity Student Study. Formate concentrations were compared with other one-carbon metabolites, vitamin status, potential formate precursors, genetic polymorphisms, and lifestyle factors. Formate concentrations were significantly higher in women than in men; oral contraceptive use did not further affect them. There was no effect of smoking or of alcohol ingestion, but the TT genotype of the methylenetetrahydrofolate reductase (MTHFR) 677CT (rs1801133) polymorphism was associated with a significantly decreased formate concentration. Formate was positively associated with potential metabolic precursors (serine, methionine, tryptophan, choline) but not with glycine. Formate concentrations were positively related to serum folate and negatively related to serum vitamin B-12. Our study statisticians have used data from our investigations for novel, important statistical research. This has continued with Drs. Wei Zhang and Aiyi Liu. Dr. Zhang has published on pooling to use genotype data in quantitative traits genome wide association studies. She has also demonstrated the value of the GATE approach to studying pleiotropic genetic associations.